1. Field of the Invention
The present invention relates to an automatic calibration apparatus for a partial gas pressure measuring sensor. More particularly, the present invention relates to an automatic calibration apparatus for a partial gas pressure measuring sensor, in which an output of a transcutaneous partial oxygen pressure measuring sensor or the like is automatically calibrated in atmospheres of at least two kinds of standard gases.
2. Description of the Prior Art
It is a widely utilized method to measure a partial pressure of carbon dioxide gas, oxygen gas or the like in blood by means of an electrochemical sensor such as a transcutaneous oxygen pressure measuring sensor. If such an electrochemical sensor is used to measure a partial gas pressure, the measurement is effected while calibration of the sensor is being done. In the following text an example of measuring a partial pressure of carbon dioxide, referred to hereinafter as PCO.sub.2, will be described. PCO.sub.2 sensor shows a fixed potential output value representing a corresponding PCO.sub.2 value. Therefore, an automatic calibration may be performed by a two-point calibration method using two different standard gases each having a known PCO.sub.2 value. Generally two air-mixed gases are used having different carbon dioxide partial gas pressures. The gas having the lower PCO.sub.2 value will be referred to as "low gas". The gas having the higher PCO.sub.2 value will be called the "high gas" in the following text.
In a conventional automatic calibration apparatus for a PCO.sub.2 sensor, it is necessary to avoid a calibration in a state in which the low gas or the high gas is not normally supplied. For this reason, a calibration using the low gas, hereinafter referred to as "low calibration", and a calibration using the high gas, hereinafter referred to as "high calibration", are performed and comparison is made output potentials of the PCO.sub.2 sensor are compared with each other. If the difference between those potentials is within a predetermined range, it is determined that the high gas and the low gas are both supplied normally and that the calibrations have been performed normally.
On the other hand, if the potential difference is outside the predetermined range, it is determined that the high gas and the low gas are not supplied normally and that the calibrations are not performed normally. Hence, the calibrations are performed again. Thus, if the high gas and the low gas are not normally supplied at the time when the automatic calibration operation is performed, it is necessary to again newly perform both calibrations. The time required for performing the automatic calibration operations is substantial and this undesirable.